app: aboot: Fix size check for boot image
app: aboot: Fix size check for boot image

If the boot image is signed include signature size while validating
the size of boot image.

CRs-Fixed: 682002
Change-Id: I8a87fd9723e87af4f17bad7635486d415a9b5a7e

M1cha/lktris,M1cha/lktris,Foxda-Tech/polaris-bootable-bootloader-lk,CanarySolutions/little-kernel,M1cha/lktris,Foxda-Tech/polaris-bootable-bootloader-lk,CanarySolutions/little-kernel,efidroid/lk,Foxda-Tech/polaris-bootable-bootloader-lk,efidroid/lk,M1cha/lktris,efidroid/lk,Foxda-Tech/polaris-bootable-bootloader-lk,CanarySolutions/little-kernel,efidroid/lk,CanarySolutions/little-kernel,CanarySolutions/little-kernel,efidroid/lk,Foxda-Tech/polaris-bootable-bootloader-lk,M1cha/lktris